The invention relates to a control circuit for the power-controlled operation of a load, comprising a semiconductor switch which is effective in a load circuit and comprising a drive circuit for the semiconductor switch, which generates a control signal, comprising drive pulses following one after the other and separated by interpulse periods, for controlling the said semiconductor switch in a part-load range.
A control circuit of this type is known from the prior art, for example DE 197 02 949 A1.
A control circuit of this type generates a control signal which has drive pulses which follow one another in a defined frequency and are pulse-width-modulated for controlling the load.
Since the switching on and off of the load brought about by the drive pulses leads in a known way to high losses, and in particular increased heat generation, especially in the case of an inductive load with a freewheeling element, i.e., for example, a freewheeling semiconductor component, it is to be regarded as the object of the present invention to improve a control circuit of the generic type in such a way that as little heat as possible is generated.
This object is achieved according to the invention in the case of a control circuit of the type described at the beginning by the control signal generating in an upper part-load range a first pulse signal, with first individual pulses following one another with a first pulse frequency, and a second pulse signal, with second individual pulses following one another with a second frequency, in the interpulse periods of the first pulse signal, and by the second frequency being greater than the first frequency by at least a factor of 10.
It is consequently to be regarded as the essence of the present invention on the one hand to drive the load, i.e. for example a motor acting as an inductive load, by the first pulse signal with low frequency in such a way that this pulse signal allows as much power as possible to be supplied to the load with as few switching-on and switching-off operations as possible, but on the other hand likewise to supply power by the second pulse signal with high frequency in the long periods between the individual pulses caused by the low frequency of the first pulse signal, in order to avoid the disadvantages of exclusively operating the load by only the first pulse signal with low frequency, these disadvantages being manifested in particular by the development of mechanical noises or the production of mechanical resonances.
With regard to controlling the power which is to be supplied to the load, a wide variety of possibilities exist here. One possibility would be to alter the frequency of the second pulse signal or, if appropriate, also of the first pulse signal.
Since, however, defined fixed frequencies are preferably used for the pulsed operation of a load to avoid peripheral disturbing influences, it is preferably provided that in the upper part-load range at least one of the first and second pulse signals can be pulse-width-modulated for power control, i.e. that the power supplied to the load can be controlled by means of varying the pulse width while the respective pulse signal has a fixed frequency.
It is particularly favourable in the case of the solution according to the invention if in the upper part-load range both pulse signals can be pulse-width-modulated, so that incremental adjustment of the pulse width of the two pulse signals allows the desired precision of the control to be achieved by setting the pulse width of that pulse signal which has the smaller increments. This is preferably the second pulse signal, it also being possible to use the second pulse signal to reduce an increase in the pulse width of the first pulse signal by a decreasing pulse width of the second pulse signal.
In the case of a control circuit operating in a particularly simple way, it is preferably provided that in the upper part-load range only one of the pulse signals can be pulse-width-modulated.
In this case, particularly precise power control can be carried out by allowing the upper part-load range to be divided into a highest upper part-load range and a normal upper part-load range and by allowing the individual pulses of the first pulse signal to be pulse-width-modulated in the normal upper part-load range and the individual pulses of the second pulse signal to be pulse-width-modulated in the highest upper part-load range.
In particular in the case of an embodiment operating in as simple a way as possible, it is at the same time provided that in the normal upper part-load range, the pulse width of the individual pulses of the second pulse signal is constant, and in the highest upper part-load range, the pulse width of the individual pulses of the first pulse signal is preferably constant.
It would be conceivable, for example, within the scope of the solution according to the invention, also to operate outside the upper part-load range with a control signal which exhibits the first pulse signal and, in the interpulse periods of the same, the second pulse signal.
However, for reasons of control simplicity and adequate precision, it is particularly favourable if, in a part-load range lying below the upper part-load range, the control signal comprises a third pulse signal with a third frequency, which is greater than the first frequency, so that the individual pulses of the third pulse signal follow one another at correspondingly small time intervals.
At the same time, the third frequency should preferably be of the same order of magnitude as the second frequency, so that both the second frequency and the third frequency are significantly above the first frequency, to allow driving of the load to be performed in the lower part-load range with as little noise and resonance as possible.
A solution which is particularly favourable on account of its simplicity provides that the third frequency and the second frequency are approximately of the same magnitude, so that the advantageous control properties of a pulse signal with a relatively high pulse frequency can consequently be utilized both in the lower part-load range and in the upper part-load range.
A particularly favourable solution provides that the third frequency is identical to the second frequency, so that ultimately the same frequency can always be used both for operating the load in the lower part-load range and for operating the load in the upper part-load range, and the second pulse signal is simply added when there is a transition from the lower part-load range to the upper part-load range.
For the power control in the lower part-load range, in this case the third pulse signal can preferably likewise be pulse-width-modulated.
The transition from the lower part-load range into the upper part-load range may in principle lie at any desired values of the part load. A particularly advantageous solution provides that the transition from the lower part-load range into the upper part-load range takes place at part-load values between approximately 20% and approximately 50%, in each case with respect to full load.
A particularly favourable solution provides that the transition from the lower part-load range into the upper part-load range takes place at part-load values of approximately 30% to approximately 40%. With regard to the differences of the second frequency and the third frequency with respect to the first frequency, it is adequate in principlexe2x80x94as already statedxe2x80x94that they are at least a factor of 10. It is particularly favourable, however, if the frequency differences have a factor of the order of magnitude of 30 or more, preferably the order of magnitude of 100 or more.
In principle it would be possible to operate in the part-load range below full driving of the load with further part-load ranges, for example also between the lower part-load range and the upper part-load range. For reasons of simplicity, however, it has been found to be favourable if the upper part-load range follows on directly from the lower part-load range.
It would additionally also be conceivable to provide outside the lower and upper part-load ranges additional part-load ranges, in which a different kind of driving of the part load can take place.
It is particularly favourable, however, if the lower part-load range and the upper part-load range cover the entire part-load range up to full load.
With regard to generating the control signals in the case of a control circuit according to the invention, no further details have been specified, in particular concerning the construction of the drive circuit. Thus, an advantageous solution provides that the drive circuit has a pulse generator and a pulse-shaping stage, it being possible for the pulse generator to be in particular a pulse generator substantially generating square-wave pulses, and the pulse-shaping stage then shaping the edges of the square-wave pulses for example in such a way that sufficiently long control times are available for the operation of the load, in particular an inductive load with a freewheeling diode.
It is particularly favourable in this case if the pulse-shaping stage generates from the square-wave pulses, rise and fall times which are time-delayed substantially in the edges.
Time-delayed rise and fall times of this type can be generated, for example, by RC elements of the pulse-shaping stage.
With regard to the generation of the first pulse signal and the second pulse signal, occurring in the interpulse periods of the first pulse signal, no further details have been specified so far. Thus, an advantageous embodiment provides that the first pulse signal and the second pulse signal can be generated as pulse signal trains having continuous individual pulses with constant frequency and that the control signal for the upper part-load range is produced from the pulse signal trains by conducting an OR operation.
This type of control signal generation can also be retained when the control signal for the lower part-load range is to be generated. In this case, for the sake of simplicity, the pulse width of the first pulse signal is reduced to substantially 0.
In addition, the object mentioned at the beginning is also achieved according to the invention by a method for the power-controlled operation of a load by means of a control circuit, comprising a semiconductor switch which is effective in a load circuit and comprising a drive circuit for the semiconductor switch, which generates a control signal, comprising drive pulses following one after the other and separated by interpulse periods, for controlling the said semiconductor switch in a part-load range, by the control signal being generated in an upper part-load range in such a way that it has a first pulse signal, with individual pulses following one another with a first pulse frequency, and a second pulse signal, with individual pulses following one another with a second frequency, in the interpulse periods of the first pulse signal, and by the second frequency being greater than the first frequency by at least a factor of 10.
It is particularly favourable in this case if, in the upper part-load range, the power control is carried out by pulse width modulation of at least one of the first and second pulse signals, the frequency of the first pulse signal and of the second pulse signal preferably being kept constant.
A particularly favourable solution with regard to the possibility of variation in this case provides that the pulse width of both pulse signals is modulated for the power control.
For reasons of simplicity, however, it is favourable if only the pulse width of one of the pulse signals is modulated, whereas the other of the pulse signals is kept constant.
Furthermore, it is of advantage for particularly precise control if the upper part-load range is divided into a highest upper part-load range and a normal upper part-load range.
Preferably, the first pulse signal is modulated with regard to the pulse width for the power control in the normal upper part-load range, while the second pulse signal is modulated for controlling the power in the highest upper part-load range.
For the sake of simplicity, the other pulse signal, respectively, in this case remains constant with regard to its pulse width.
In addition, it is of advantage for controlling the power outside the upper part-load range if, below the upper part-load range, a control signal which comprises a third pulse signal with a third frequency, which is greater than the first frequency, is generated.
In this case, the third frequency is preferably likewise chosen such that it is of the same order of magnitude as the second frequency.
For reasons of simplicity, however, it is particularly favourable if the third frequency is identical to the second frequency.
Furthermore, it is likewise of advantage for controlling the power in the lower part-load range if the third pulse signal is modulated with regard to its pulse width, the frequency with which the individual pulses follow one another in the case of the third pulse signal likewise being kept constant in particular.
With regard to the position of the upper part-load range and of the lower part-load range in relation to one another, so far likewise no specific details have been specified. The upper part-load range and the lower part-load range could, for example, still be separate from one another. It is particularly favourable, however, if the upper part-load range follows on directly from the lower part-load range.
Furthermore, it is preferably provided for reasons of simplicity that the lower part-load range and the upper part-load range cover the entire part-load range up to full load.
To allow the control signal to be generated as simply as possible in the case of the method according to the invention, it is preferably provided that the first pulse signal and second pulse signal are generated as continuous pulse signal trains with constant frequency and the control signal in the upper part-load range is generated from the two pulse signal trains by conducting an OR operation.
In the same way, there is also the possibility of generating the control signal in the lower part-load range, the pulse width in this case being kept substantially at 0.
Further features and advantages of the invention are the subject of the following description and the graphic representation of an exemplary embodiment, in which: